1. Field of Invention
The present invention relates generally to the field of heavy oil and bitumen recovery, and more specifically to compositions and methods of use thereof for production of heavy oil and bitumen from reservoirs containing same.
2. Related Art
Asphaltenes are the heaviest, most polar fraction of bitumen and are defined as the fraction, which are soluble in aromatic solvents such as toluene or benzene and precipitate from an oil sample upon the addition of n-alkanes such as n-heptane or n-pentane. Due to the complex structure of asphaltenes, it has proven difficult to establish the nature of asphaltenes in crude oil. This is partly because asphaltenes are a solubility class and not a pure component. They consist of tens of thousands of chemical species and the composition is not well defined. In addition, they appear to interact with each other and the other oil constituents in a complex manner. It has been proposed that asphaltenes exist as colloidal particles (Pfieffer and Saal, 1940; Dickie and Yen, 1967), as micelles or reverse micelles (Storm and Sheu, 1994) or as macromolecules in a non-ideal solution (Hirshberg et al., 1984). These proposed structures lead to different modeling approaches based on the identified structure of asphaltenes in crude oils.
In a stable crude oil, asphaltenes are in thermodynamic equilibrium with other components of the oil. However asphaltenes may precipitate due to disruption of this thermodynamic equilibrium due to a change in pressure, temperature or composition of the system. In other word composition and operational changes may affect the solubility of asphaltenes in oil and cause the asphaltenes to precipitate. In order for a liquid (oil) to dissolve a solute (asphaltene), the interaction between liquid and solid molecules should be as strong as the interaction between liquid molecules. Solubility parameter, or rather the difference in solubility parameters, is a good measure of the solubility of a solute in a solvent. Solubility parameter is a type of cohesion parameter, which describes the interaction between molecules in condensed material. However, solvents with similar solubility parameter may have different solvating power due to the nature of their molecules. This distinction is best described by Hansen solubility parameter (Hansen et al., 1967), which is a combination of dispersive, polar, and hydrogen bonding solubility parameters accounting for the nature of the molecules. It is also important to note that the solubility parameter of a liquid mixture is proportional to the amount of each liquid assuming the two liquids are completely miscible. Having this property in mind, a cocktail of solvents may be prepared to have a specified solubility parameter to ensure solvency of the solute, while providing other properties such as relatively lower surface tension for viscosity reduction.
Light hydrocarbons, such as propane, butane, and the like, are known to induce not only asphaltene precipitation but also resin instability upon mixing. Aromatic compounds, such as toluene, xylene, and the like, as well as some other solvents like CS2 are true asphaltene solvents. In other words, asphaltene solids dissolve in these solvents.
Heavy oil and bitumen are very viscous and difficult to recover from the reservoir. One way to reduce the viscosity of heavy oil is injection of hydrocarbon vapors, such as n-butane at its dew point pressure through a horizontal well. The diluted oil is then produced from another horizontal well beneath the injector by gravity. This method is called vapor extraction or “vapex.” Vapor extraction has been studied on a laboratory scale for more than a decade, but commercial application of this method has been limited. One of the limiting factors is solid asphaltene precipitation and deposition in the reservoir and/or flow lines. The main disadvantage of previously proposed vapor extraction processes is the precipitation of asphaltenes during the course of production in the reservoir causing not only wettability alteration of reservoir rock, but also a more severe production problem which is the plugging of reservoir flow channels (also known as reservoir pores). This is known in the art as formation damage. If plugged, the reservoir cannot produce the the otherwise recoverable oil in place.
Thermal methods such as steam injection and cyclic steam stimulation have been used for heavy oil recovery for more than 50 years. However, thermal methods (involving steam) are being increasingly challenged by reservoir conditions and environmental issues. For thin reservoirs, heat loss to the adjacent formation may be prohibitively high. Reservoirs with underlying aquifer, fracture, or sensitive clays are also not suitable for steam injection. Another limiting factor is the reservoir depth. If the reservoir is too deep, the steam temperature has to be high, which results in higher heat loss and causes more mechanical problems for the tools. A more serious issue is the use and contamination of water, which otherwise would be useful for agriculture or other civil purposes. Consequently, tighter legislations are emerging to control/curb water use for the energy industry.
Steam injection for heavy oil recovery may eventually become unattractive due to either lack of suitable reservoirs or environmental constraints/regulations. An alternative to thermal recovery processes is, therefore, necessary. The above-described vapor extraction process is a variant to the steam assisted gravity drainage (SAGD) process but it has numerous implementation problems as discussed above, namely, solid asphaltene precipitation and/or deposition in the reservoir and flow lines. Thus, a practical approach is required to reduce/mitigate the adverse effects of precipitated/deposited asphaltenes while using a vapex process.
U.S. Pat. No. 6,357,526 (Abdel-Halim, et al), discusses field upgrading of heavy oil and bitumen. Other patents of interest include the following patents assigned to Ormat, Inc., related to crude oil deasphalting technology: U.S. Pat. Nos. 5,804,060; 5,814,286; 5,843,302; 5,914,010; 5,919,355; 5,944,984; 5,976,361; 6,183,627; 6,274,003; 6,274,032; 6,365,038.
U.S. Pat. No. 5,100,531 discusses an asphalt or asphaltene refinery anti-foulant technique comprising the use in crude oil, or crude oil fraction, streams of alkyl-substituted phenol formaldehyde liquid resins in combination with hydrophilic-lipophilic vinylic polymers. The polymeric anti-foulant is stated, when added to asphalt or asphaltene containing crude oil streams, to prevent fouling of metallic, or other, i.e. ceramic, surfaces, especially heat transfer surfaces.